Liquid-Liquid Base Extraction of Benzoic Acid from Acetanilide followed by
Recrystallization/Product Identification by Melting Point Determination
March 23, 2007
Methods and Background
The purpose of this experiment was to separate a 1:1 mixture of benzoic acid and acetanilide
through extraction with aqueous sodium hydroxide and purify the products by recrystallization.
Liquid-liquid extraction is used to remove an organic compound from a mixture, and it involves
the partitioning of compounds between two immiscible phases (phase distribution), which is
governed by selective dissolution.
In this technique, a solute,
, is dispersed between
extracting phase, and
, the original phase; typically the immiscible liquids used are water and
an organic solvent, such as dichloromethane.
In liquid-liquid extraction, the two immiscible
liquids essentially compete for solute
, and once equilibrium has been established, the ratio of
in each phase will remain constant.
, the partition coefficient, expresses the
, in g/mL, in each phase, and it is approximated by dividing the solubility of
by the solubility of
in the original solvent
is greater than 1,
be mostly in
Furthermore, the proportion of
after multiple extractions
equal to the volume of original solvent
divided by the quantity of
multiplied by the volume
of extracting solvent
, with the whole expression raised to
, the number of extractions
From this equation, it can be deduced that multiple extractions become significant as
the value of
decreases to increase the quantity of solute extracted.
When choosing a suitable
extracting solvent, some requirements must be kept in mind: it must not be chemically reactive
with the components of the mixture, it must be immiscible with the original solution, it must
have a higher
for the desired component being extracted, and it should be separable from the
Carboxylic acids contain functional groups that are polar (due to interatomic electronegativity
differences), and unless they contain fewer than about six carbon atoms, they will typically be
insoluble or only slightly soluble in water (
is less than 1) due to the hydrophobic properties of
the portion of the molecule containing carbon (R or Ar); however, carboxylic acids are soluble in
organic solvents such as dichloromethane, which have moderate polarity, so if they are extracted
with water, they will generally remain in the organic solvent phase.
The amino functional group
is also polar and hydrophilic, and amines possessing more than six carbon atoms are insoluble or